(1) Field of the Invention
The present application relates to a protective and abradable coating composition for application on rolls and more particularly to conveyor rolls comprising an abradable coating for use at high temperature applications, to a composition for producing such coaling, to a process for making such rolls and to the use thereof.
(2) Description of the Related Art
Conveyed metal sheets are often treated against corrosion with aluminium or zinc alloys containing aluminium. Rolls made of materials comprised of fused silica, mullite, sillimanite, alumina, SiC matrix or metal such as refractory steel are well known for their excellent behavior at high temperature such as their high refractoriness and low thermal expansion and for their relative inertness with respect to molten metals. They are widely used in roller hearth furnaces. Although rolls containing silica are preferred, they present a drawback: while progressing in the furnace, the aluminum of coated metal sheets starts to melt. Partially melted aluminum reacts with SiO2 or silicates issued from the ceramic roll body leading to aluminum oxidation (Al=>Al2O3) and silicate reduction to metal form (mainly Si), reducing then drastically the life of the rolls due to corrosion.
It is known that, in order to prevent corrosion, rolls can be coated with SiAlON or with sintered silicon nitride or silicon carbide. The coating can further comprise particles of aluminium titanate. All these materials are indeed known to have non-wetting properties with respect to molten aluminium.
Even though the corrosion is reduced, some build-up phenomena still persist. By using a hard or durable silicon nitride coating, Si3N4 is slowly oxidized at the surface resulting in the formation of silica. Minor phases, such as silica coming from silicon nitride oxidation, react with aluminum producing Al2O3 and Si which accumulate on the coaled roll surface.
A hard coating disclosed in US-A1-2007/089642 comprises silicon nitride particles and a binder comprising surface-modified nanoscale solid particles in an organic solvent. The coating is described as hard due to the strong adherence of the nitride coating to the substrate. An organic solvent is used to avoid any hydrolysis of silicon nitride particles. This hard coating is usually applied on solar crucibles but can be also applied on riser tubes in aluminum metallurgy to prevent the aluminum corrosion.
US-A1-2009160108 describes fused silica roll without coating, where the surface porosity has been filled for example with silicon nitride using an organic solvent. This prevents to some extent the surface build-up. As fused silica is still the major phase on the surface, corrosion is also observed when in contact with aluminum.